Implementing GREP command using System calls

Ex. No.: 4(a)(ii)

Implementing GREP command using System calls

AIM:

            To write a C program to implement the UNIX command grep.        

Syntax

	grep [options] PATTERN [FILE...]

 

Description:

            Grep searches the named input FILEs (or standard input if no files are named) for lines containing a match to the given PATTERN.  By default, grep prints the matching lines.

Algorithm

1. Start of Algorithm
2. Get the command as the command line argument and check for the syntax of the command.
3. Check for the existence of the file if not exists gives the error message.
4. Open the file if exists.
5. Read the file contents line by line and check for the pattern matching if match occurs print the line.
6. End of algorithm.

Program:

#include <string.h>

#include <fcntl.h>   //Defines flags for open and related library functions

#include <sys/uio.h>

#include <unistd.h>

main(int argc, char *argv[])

{

     int des, fhandle, i,k, arg_ind=2;

     int linecount;

     int lineflag;

     char buf,line[160];

     if (argc < 4)  // To check the syntax

    {

       printf( "\n Error... Correct Syntax is : grep  \"pattern\" \"filename\"\n");

        exit(0);

     }

     lineflag=0;

     des = open(argv[3],O_RDONLY);     //Open the file specified by the command

     linecount=0;

       do          // to read upto the end of file and copy to the array line[].

       {

          i=0;

        do       // to read a line

          {

              fhandle=read(des,&buf,1);

              line[i++]=buf;

           } while(( fhandle!= 0) && (buf != '\n'));

           line[i]='\0';

            linecount++;

            i=0;

         while(line[i] != '\0')        // to check character matching

          {

            if(line[i] == argv[1][0])

             {

                //continue checking till the end of pattern

               k=1;

               i++;

               while((k < strlen(argv[1])) && (line[i] == argv[1][k]))

               {

                    i++; k++;

               }

               if(k == strlen(argv[1]))

              

                      printf("%s",line);

                       }

           i++;

         }

      }while( fhandle!= 0);

close(des);

      arg_ind++;

      printf( "\n");

    }

Output:

$ cat input

Operating System Services are

Process management

Main-memory Management

File Management

I/O System Management

Secondary Storage Management

$ ./a.out grep Management input

Process management

Main-memory Management

File Management

I/O System Management

Secondary Storage Management

RESULT :

            Thus the program to simulate the UNIX command grep was written and executed successfully.